1. Field
This invention relates generally to methods, systems and apparatus for managing digital communications systems. More specifically, this invention relates to a DSL system using ring signal conditioning, impedance conditioning and/or other similar device, method or technique to improve high-speed communications.
2. Description of Related Art
Digital subscriber line (DSL) technologies provide potentially large bandwidth for digital communication over existing telephone subscriber lines (referred to as loops and/or the copper plant). Telephone subscriber lines can provide this bandwidth despite their original design for only voice-band analog communication, referred to as Plain Old Telephone Service (POTS). In particular, asymmetric DSL (ADSL) and very-high-speed DSL (VDSL) can adapt to the characteristics of the subscriber line by using a discrete multitone (DMT) line code that assigns a number of bits to each tone (or sub-carrier), which can be adjusted to channel conditions determined during initialization and subsequent on-line training known as “bit-swapping” of the modems (typically transceivers that function as both transmitters and receivers) at each end of the subscriber line. Many consumer telephone lines have network electronic devices installed at or near the entry to the customer premise that are designed to provide network maintenance, diagnostic and/or protective functions. These devices have been installed by the telephone companies and are not accessible for modification by the customer or third-party service providers.
While telephone systems provide spectrum separation for POTS and DSL service, such services' co-existence still is complicated by the fact that when POTS was originally defined, only voice-band-type communications and equipment was considered. Consequently, specifications and standards for compatible equipment were very strict in the 0-4 KHz range, but almost non-existent beyond that range. Thus, POTS equipment and operation can produce detrimental interference to DSL systems using higher frequencies and running on the same telephone line/loop as POTS. The result of such interference can include reduced performance (for example, reduced data rates, increased error problems, etc.) and, in some circumstances, data loss or DSL modem disconnection. All these are challenges for successful DSL operation on lines concurrently providing POTS.
Appropriately placed high-pass and low-pass filters (networks that also are referred to as splitters) are utilized to isolate the function of the low frequency POTS service from the function of the high frequency DSL service. At consumer premises, splitters can be 3-ported entities or apparatus—one port for the phone line, a second port for the telephone, and a third port for the ADSL modem. In some situations the splitter is divided into two 2-port devices, one of which is a high-pass filter placed in the ADSL modem, and the second lowpass 2-port, often known as a “micro-filter” interfaces the phone line to the telephone. A splitter in either realization is designed to pass high frequencies (for example, greater than 8 KHz) with little or no attenuation between the line and ADSL port, and to pass low frequencies (for example, below 4 KHz) with little or no attenuation between the line and phone ports. In theory, only linear devices will be attached to the telephone line in which case inclusion of splitters at both the telephone company central office (CO) and consumer premises allows non-interfering operation of POTS and DSL. If a splitter is incorrectly installed or missing altogether at the subscriber premises, the presence of POTS signals, either ringing or voice, can impair the DSL service on the shared line.
However, certain telephone equipment which was not originally designed for DSL operation can introduce non-linear distortion to the phone line. In such circumstances the splitters, which rely on linear system behavior, cannot isolate the POTS line causing detrimental behavior to the DSL service.
A primary source of such POTS interference is triggered by POTS ringing. POTS ringing can cause certain devices to exhibit non-linear behavior because of both high voltages as well as long duration of impulses during the ring period. These devices such as the 6PT in Japan or, more generally, Metallic Distribution Units (or MDUs) or surge protectors located just outside the customer's premises may be inadvertently triggered into modes of non-linear behavior. This non-linear behavior introduces non-linear harmonics and impulses (consequent to ringing) that are on the wrong side of the micro-filter or splitter and thus introduce distortion and impulse impairments directly into the DSL signal. These impulse disturbances are not then reduced at all by the splitter and disrupt DSL service. Conventional approaches to curing the resulting DSL bit-error problems, such as FEC (forward error correction) and interleaving, are helpful, yet still are often not sufficient to reduce problems caused in DSL by POTS interference. The same is true of the situation where a splitter is incorrectly installed or altogether missing.
A 6PT box is a device inserted in (that is, coupled to) Japanese telephone lines to prevent those telephone lines from being affected by abnormally high external voltage, as well as to isolate the customer premises from the network to allow fault isolation. Because the 6PT was not originally designed for DSL compatibility, it can cause problems in DSL service on the same telephone line when the telephone rings. While some 6PT boxes have additional problems due to faulty design, all 6PT boxes have a non-linearity problem that is not a faulty design, it simply is that DSL was not considered when the 6PT devices were designed and installed. The telephone ringing voltage can trigger non-linear loads within the 6PT and/or activate the 6PT device and unintentionally interrupt the DSL service.
Some earlier systems change the DSL signal to avoid the non-linear interruptions. For example, DSL service is suspended or ceased during ring-signal presence. Such a solution, however, is incompatible with many DSL service functions, such as streaming video, music, gaming, etc. Other solutions today include tolerating data errors during incoming ring signals or hiring an authorized network service technician to replace the incompatible network electronic device with a different unit, both of which are undesirable. Moreover, present methods of detection of such a device's presence rely on telephone company records, which are often inaccurate and/or incomplete.
Systems, devices, methods, techniques, etc. that permit normal POTS operation, including telephone ringing, without impairing or causing disconnection of DSL service on the same telephone line in installations that include 6PT, MDU or similar network devices that cause ring signals and their generated impulse disturbances to impair the DSL service would represent a significant advancement in the art. Moreover, systems, devices, methods, techniques, etc. that permit detection of 6PT, MDU and/or other similar network devices likewise would represent a significant advancement in the art. Similarly, systems, devices, methods, techniques, etc. which could analyze the impairments of a 6PT, MDU and/or other similar network device, identify the presence of such devices and/or prescribe the optimal form of a remedy would represent a significant advancement in the art.